The combining of two or more liquids together to form a defined mixture of the constituent liquids is fundamental to many industrial processes and commercial products. This combining of liquids may be referred to as batching or blending and is common to many industrial segments including pharmaceutical products, biomedical products, food processing products, household products, personal care products, petroleum products, chemical products and many other general industrial liquid products.
Most typically, liquid products are made by combining relatively large quantities of each constituent. Constituent liquids are held in large tanks and are moved in correct volumetric or weight ratio into another large tank where mixing of the liquids occurs. This general process is referred to as batching.
Liquids batching entails many drawbacks and limitations. For example, large tanks are required and large volume batches are typically prepared. This large scale requires substantial manufacturing space, and large batch volumes dictate a relatively fixed and inflexible manufacturing schedule. Large volumes are typically batched in order to overcome the relative imprecision of constituent liquids measurement. Large volumes help to reduce these errors as a percentage of the total batch quantity. Another drawback of batching is that the batch frequently changes its rheological or chemical properties over time. This aging effect is common to many formulations and over time it forces many adjustments to be made in the packaging machines used to fill the batched liquids into unit of use containers. Batching also can lead to open or partially open tanks and to liquids exposed to the atmosphere. This can lead to unwanted chemical degradation and to microbial contamination.
Batching can also lead to difficulties in mixing together the liquid components in large volumes. It is often true that the components can be mixed only with difficulty and require prolonged agitation to become homogeneous. It is also well known that it is common for different levels of a large tank to have different proportionate mixtures of the liquids. It is also true that the large volumes typically committed to batching cause cleaning to be slow, laborious and difficult to automate. Large volumes of cleaning effluents are produced, leading to issues of waste and contamination.
Because of these numerous and substantial shortcomings and limitations, alternative means of liquid products manufacturing have been sought. One alternative method to batching is termed continuous stream blending.
Continuous stream blending embodies the notion of combining constituent liquids to form a liquid product only as needed or on a demand basis. Essentially, product is made only as required and at the rate required. The rate required is typically based on the demand of the liquid filling machine packaging the liquid product.
The appeal and merit of a continuous stream blending system, as distinct from a batching system, is clear. The ability to eliminate large liquid product batch preparation and holding tanks leads to a small system volume, more product compounding flexibility, faster product species turn around, smaller and shorter practical packaging run capabilities and a substantially lower capital asset commitment. Continuous stream blending can also yield superior product formula accuracy and quality, and can eliminate the barrier or "wall" between liquid products processing and liquid products packaging, as well as greatly reduce waste, cleanup time and effluent volumes. Furthermore, mixing is simplified and product aging effects are largely eliminated. The real issue is how to build a continuous stream blending system with the maximum degree of accuracy, flexibility of use, and versatility of application in a broad range of commercial sectors.
Numerous designs for continuous stream blending have been set forth, originating from various liquids processing industries, particularly beverage processing and food processing. These designs have been attempts to develop and market continuous flow proportioning or blending systems based upon ratio flow control using flow meters and proportioning--integrating--derivative (hereinafter--PID) feedback control loops. For example, H & K Inc. of New Berlin, Wis., has introduced "Contimix" based upon this design approach. In general, these designs rely on regulating a continuous flow of the liquid streams using variable orifice valves or speed controlled pumps, where the flow rate signal from a flow meter, most often a Coriolis mass flow meter, is used to proportionately modulate the flow control device in order to attempt to maintain a desired ratio of flows among the streams, and where another signal representing overall system demand rate is used to proportionately modulate the summed flow of the entire system.
Several major design problems are encountered with continuous flow blending systems utilizing this flow architecture. First, as the overall output of the system is increased or decreased, the rate of change capability or response time constant of each stream will vary one from the next. Thus, with a varying output command signal, each stream reacts at a different rate causing loss of ratio flow and this is further aggravated by the overshoot or undershoot of each stream as a new set point is reached. Also, as each stream flow rate changes it can perturb the flow rate of the other stream or streams causing hunting or oscillations. These common control problems can cause serious loss of blended stream accuracy. Clearly, PID loop controllers are designed to "tame" or control complex systems which are not inherently designed for stability or ease of control. They deal with the interacting multiple dependent and independent variables of a flow stream in a non-real time, statistical way and "fight" changing parameters on an historical basis.
Still another problem can arise when a feedback signal change causes the flow to briefly go below or above the permissible range of the flow meter generating the feedback signal. Even with software or hardware safeties this can occur and, as will be discussed fully further on, the requirement to maintain flow through a Coriolis mass flow meter within a defined range to achieve satisfactory accuracy is clearly demonstrable.
Perhaps the major problem encountered with these designs and the PID control architecture arises with the inevitable need to start and stop the flow stream system. When a stop-start event occurs, it is very difficult to bring the system back on-line with balanced (that is to say accurate) flow and blending. This problem has been so persistent that nearly all installed systems have resorted to the use of a surge tank of up to several hundred gallons capacity to allow blending flow to continue during brief filler machine stoppages.
Even with the use of a surge tank, if blending flow must stop because of a prolonged filler stoppage, upon re-start the flow streams must either be diverted until correct flow rates are reestablished, or the surge tank must be quite large to allow poorly matched flow ratios to be statistically "diluted" to prevent loss of accurate blending. Either method results in substantial waste, decreased blending accuracy, increased system complexity and increased system volume, thus depleting the sought after advantages of continuous stream blending.
Another major problem encountered with PID control designs occurs when blending flow rates must be altered to adjust for variable take-away demand. When this occurs, the system is "perturbed" and the flow rates of each feed stream must be slewed or varied. This can occur at only a finite and limited rate, and the rate of change in flow rarely matches on each flow stream. The result of these inherent limitations is a lag in response to flow rate change commands, unbalance in flows during change, and overshoot of the new flow set point. These phenomenon are inherent to PID based schemes and limit the overall performance of such systems.
While it is possible to "tune" PID controls in order to minimize dynamic control limitations and inaccuracies, in a continuous stream blending system designed to operate as a general purpose device across a wide range of blend formulas, liquid properties and flow rates, a PID loop control scheme cannot be readily optimized.
The prior art discloses several examples of designs which use Coriolis mass meters to control liquid blending for various particular purposes.
Clem, 5,325,852, discloses a means of combining a liquid and a gas in order to alter the density of the liquid. The liquid density and mass flow are established using a Coriolis mass meter, while the flow rate of the gas is regulated by a thermal dispersion type gas mass flow meter. Analog signals from the flow meters are used for control purposes. This patent does not disclose method or apparatus for combining continuously flowing multiple liquid streams on a mass ratio basis.
Clem, 5,481,968, discloses a means and apparatus to adjust the densities of liquid feed streams using Coriolis mass meters as densitometers, and provides for combining streams of liquids to achieve a desired final density. Mass meters control stream flow rates via a PLC feedback loop to proportionately control an adjustable speed pump drive for the purpose of adding one stream to another stream.
In 5,656,313, Gibney et al. disclose a beverage syrup blending apparatus in which the mass flow of a liquid component is determined by a Coriolis mass flow meter, the mass flow converted by formula into a volumetric flow, the volumetric flow signal then being used to proportionately control the ratio of flow of two liquids, control being by adjustment of a variable orifice liquid flow control valve.
In a commercial publication issued by H & K Inc., New Berlin, Wis., there is disclosed a continuous blending system named Contimix. The document describes a multiple liquid stream apparatus in which Coriolis mass meters proportionately control the continuous flow rate of the streams to formula defined mass ratios. The liquid streams combine into a common manifold.